1. Field of the Invention
The present invention relates generally to valves which employ a stuffing box packing system and more particularly to load profile compensation in such a system.
2. Background Art
The current state-of-the-art of a stuffing box is a cylindrical box rectangular in cross-section filled with rectangular cross-section packing rings (typically a quantity of five rings per set). Through this packing protrudes a valve stem with a linear (up and down) motion. This packing is held in place by a packing gland with a register that fits into the stuffing box. This gland is held in place with fasteners that hold it to the valve bonnet. Furthermore, this packing has to be tightened to create a seal between the stem and packing and the packing and the stuffing box bore. The amount of axial load that is applied to the packing through the gland has to be sufficient to seal against the valve internal pressure. This axial load when applied, translates to a radial packing load to provide the sealing capability on the packing inner and outer surfaces. Radial packing load is at a maximum at the junction of the gland and the top packing ring and it decreases exponentially toward the second ring and below. Therefore, in reality most of this packing sealing is done by the top ring. Ideally, for service life it would be advantageous if the packing load can be transmitted uniformly through the entire group of rings without any reduction in this load to provide uniform wear.
When tightening a rectangular cross-section, graphitic packing, load is applied by tightening the valve gland bolts which exerts load on to the gland flange and thereby to the packing. The applied load acting in a vertical axis parallel to the stem and within the annulus of a valve stuffing box compresses initially the top ring of packing then successive rings with regard to an individual die formed packing ring. This compression causes the middle of each of the packing to bulge and eventually contact the stem and the walls of the stuffing box. Therefore, force from the gland bolts is transmitted into compressive pressure on the stem and stuffing box walls from the above described xe2x80x9ctransmission process.xe2x80x9d There is loss in transmitting this mechanical work. Such losses through friction and densification compression consolidation of the graphitic packing itself.
For 100% dense materials, Poisson""s ratio (ratio of height change to related lateral expansion or contraction) is approximately 0.3 or 33% in the best case. However for flexible graphite packing materials for which 70-90 lb/ft3 density is typical, flexible graphite is approximately 50-65% theoretical density. Therefore, when a flexible graphite packing is compressed, the work of axial compression is divided between increasing the local density of the packing and Poisson""s expansion of the packing in the direction transverse to the application of force, i.e., radial load.
The present invention provides several alternative embodiments to uniformly distribute the radial packing load. As stated earlier, the last four of the conventional rings do a minimal amount of sealing, but they do add to the packing friction which, in turn, requires additional operator torque to move the valve stem. It is logical to consider, therefore, using only one packing ring and using this ring more effectively, which, in turn, reduces packing friction, operator torque, stem wear (corrosion, pitting, wear and tear) and lower maintenance costs by minimizing leaky packing replacements.
In a first embodiment of the invention the cylindrical wall of the stuffing box is conically tapered with a flat bottom. This stuffing box configuration is to be designed to accept rectangular cross-section packing. This configuration makes the radial packing load uniform throughout the packing ring elevation.
In a second embodiment the cylindrical wall of the stuffing box is straight with a tapered outbound bottom. This stuffing box configuration is designed to accept square cross-section packing. This configuration makes the radial packing load uniform throughout the packing ring elevation.
A third embodiment is similar to the second except the bottom of the packing ring is tapered at the same outbound angle to match the outbound taper at the bottom of the cylindrical stuffing box.
In a fourth embodiment the cylindrical wall of the stuffing box is straight with a tapered inbound bottom. This stuffing box configuration is designed to accept square cross-section packing. This configuration makes the radial packing load uniform throughout the packing ring elevation.
A fifth embodiment is similar to the fourth except the bottom of the packing ring is tapered at the same inbound angle to match the inbound taper at the bottom of the cylindrical stuffing box.
In a sixth embodiment the cylindrical wall of the stuffing box is straight with a male inverted xe2x80x9cVxe2x80x9d x-section at the bottom of the stuffing box. This stuffing box configuration is designed to accept a packing ring that is flat on the top, straight on the sides and has a female inverted xe2x80x9cVxe2x80x9d groove on the bottom. The bottom on the packing ring should mate with the bottom of the stuffing box. This configuration makes the radial packing load uniform through the packing ring.
It is therefore a principal object of the present invention to provide a non-rectangular cross-section stuffing box packing system for valve stems to result in a uniform radial load profile through the packing ring and thus increase sealing efficiency, reduce wear and maintenance.
It is another object of the invention to provide a tapered packing ring and stuffing box cross-section to more uniformly distribute radial forces and enhance service life by reducing local wear patterns as would be the result of non-uniform loading applied by the packing ring against the valve stem.
It is still another object of the invention to provide an improved stuffing box packing system for sealing valve stems, the improvement being in the cross-sectional shape of at least the stuffing box to more evenly distribute radial loading throughout the height of the packing ring.